1. Field of the Invention
The present invention relates generally to passive tracking and, in one possible embodiment, relates to a two-cluster angle of arrival (AOA) tracking system wherein data from asynchronous ultrawideband (UWB) receivers are utilized to estimate time difference of arrival (TDOA) of UWB pulses utilizing a cross-correlation peak detection (CCPD) method.
2. Description of Related Art
While GPS and radar tracking systems are often very useful, GPS and radar tracking systems are not suitable or available for all tracking jobs. As one example, GPS systems are not currently available to provide long-range line of sight tracking of lunar or Mars rovers and astronauts. Other examples may include situations with a need for fine tracking accuracy but where GPS does not operate well and/or where radar systems may cause undesired emissions or interference.
Prior art AOA tracking systems use antenna arrays to extract phase information from a continuous sinusoidal signal, and then convert phase information to angle information. However, prior art AOA tracking systems have significant problems that leave them unsuitable for many tracking problems. The resolution of prior art AOA tracking systems is relatively low, which results in large tracking errors. In many prior art AOA systems and/or other prior tracking systems discussed hereinafter, synchronization between the transmitter and receivers is required, which may give rise to inaccuracies due to the synchronization error. For example, one microsecond synchronization error may linearly translate into an unacceptably 300 meter ranging error. Such systems may also cause interference with other electronic equipment and/or be affected by such interference. Moreover, such systems are likely to be adversely affected when operated in an electrically noisy, multipath environment.
The following patents show prior art efforts regarding the above and other problems:
U.S. Pat. No. 5,920,278 issued Jul. 6, 1999, to Tyler et al, discloses a broadband transmitter element, located at a remote object, which transmits a broadband signal at a prescribed transmission time. A broadband receiver element, located at a base platform spaced from the remote object, receives electromagnetic radiation during a reception search window. The broadband receiver element stores information characterizing the broadband signal. A synchronizer synchronizes the broadband transmitter element with the broadband receiver element for timing the transmission and reception. A processing device derives an estimated time of flight for the broadband signal to travel from the remote object to the base platform, and a correlation detector, located at the base platform, identifies the remote object and the arrival time of the broadband signal by correlating the stored information with signals received during the reception search window.
U.S. Pat. No. 6,760,387 issued Jul. 6, 2004, to Langford et al, and corresponding US Patent Publication No. 2003/0058971, disclose a system and method for determining angular offset of an impulse radio transmitter using an impulse radio receiver coupled to two antennas. The antennas are separated by some known distance, and, in one embodiment, one antenna is coupled to the radio with cable delay. Impulse signals from the antennas are measured to determine the time difference of arrival of one such signal received by one antenna compared to that of the other antenna. Time differential is measured by autocorrelation of the entire impulse radio scan period, by detecting the leading edges of both incoming signals or various combinations of these methods. Using a tracking receiver, the pulses may be continuously tracked thus providing real time position information.
U.S. Pat. No. 6,882,315, issued Apr. 19, 2005, to Richley et al, and corresponding US Patent Publication No. 2004/0108954, disclose an RF object locating system and method that uses or includes a set of N (N>2) receivers (monitoring stations) located at fixed positions in and/or about a region to be monitored, one or more reference transmitters that transmit a timing reference, a location processor that determines object location based on time-of-arrival measurements, and at least one object having an untethered tag transmitter that transmits RF pulses, which may additionally include object ID or other information. Free-running counters in the monitoring stations, whose phase offsets are determined relative to a reference transmitter, are frequency-lacked with a centralized reference clock. Time-of-arrival measurements made at the monitoring stations may be stored and held in a local memory until polled by the location processor. The invention permits acquisition of tag transmissions.
U.S. Pat. No. 6,111,536, issued Aug. 29, 2000, to Richards et al, discloses a system and a method for distance measurement utilizing a radio system. The distance is measured by determining the time it takes a pulse train to travel from a first radio transceiver to a second radio transceiver and then from the second radio transceiver back to the first radio transceiver. The actual measurement is a two-step process. In the first step, the distance is measured in coarse resolution, and in the second step, the distance is measured in fine resolution. A first pulse train is transmitted using a transmit time base from the first radio transceiver. The first pulse train is received at a second radio transceiver. The second radio transceiver synchronizes its time base with the first pulse train before transmitting a second pulse train back to the first radio transceiver, which then synchronizes a receive time base with the second pulse train. The time delay between the transmit time base and the receive time base can then be determined. The time delay indicates the total time of flight of the first and second pulse trains. The time delay comprises coarse and fine distance attributes. The coarse distance between the first and second radio transceivers is determined. The coarse distance represents the distance between the first and second radio transceivers in coarse resolution. An in-phase (I) signal and a quadrature (Q) signal are produced from the time delay to determine the fine distance attribute. The fine distance indicates the distance between the first and second transceivers in fine resolution. The distance between the first and second radio transceivers is then determined from the coarse distance and the fine distance attributes.
US Patent Publication No. 2006/0105776, published on May 18, 2006, by Burke, discloses an emergency service architecture for determining the location of a wireless caller. The architecture leverages a synchronization feature of GSM networks of the Base Station Subsystem (BSS) to enable employment of sparse networks by removing WLS (Wireless Location Sensor) equipment from selected cell sites. Thus, the location of a wireless caller within a sparse site can be determined. Sparse network location services can be provided further utilizing Time Difference of Arrival (TDOA) technology, and other network-based location technologies such as Enhanced Observed Time Difference (EOTD) and Angle of Arrival (AOA). Hybrid network-based/handset-based location technologies may also be used with the disclosed invention.
US Patent Publication No. 2006/0046687, published on Mar. 2, 2006, by Kwon, discloses an apparatus and a method for transmitting/receiving emergency rescue signals. In the apparatus and the method, when an emergency occurs, it is possible to provide a function capable of notifying a place at which such an emergency has occurred by means of a corresponding terminal. Accordingly, when a rescue request is accepted, a rescue center sends a rescue team to the general position of the terminal based on the GPS position information or the reporter's statement, and finds out an exact point for a corresponding terminal by means of a searching apparatus. Herein, the rescue team receives UWB signals received from the corresponding terminal of a victim at the already understood general position through the searching apparatus, and finds the victim's exact position.
An article entitled “Position Location for Indoor UWB Systems,” by Yi-Ching Yeh, discloses: An indoor geolocation system for commercial, public safety, and military applications. Since most wireless communication systems used for indoor position location may suffer from dense multipath situation, which leads to a severe degradation of position accuracy. The TDOA/AOA (Time Difference of Arrival/Angle of Arrival) position location for indoor ultra-wide band (UWB) systems in the thesis uses resolution of UWB signals. In the line of sight situation, by means of increasing angle of arrival (AOA) information to time difference of arrival (TDOA) based location to achieve the goal of accurate indoor geolocation and provides non-line of sight (NLOS) error mitigation for time measurement and AOA selection to suppress the impact to position accuracy in NLOS environment. Finally, the extended Kalman filter is used to perform position tracking of the target. In the simulations, the NLOS error in time measurement is produced according to the characteristics of indoor UWB channel. Several assumptions of NLOS errors are made in angular measurement. It is observed that proposed method efficiently mitigates the position error in NLOS environment, and detects if the NLOS exists between base station and mobile station immediately.
The above approaches are limited in various ways. They utilize features that may require complex electronics and/or give rise to errors, and/or rely on satellites. For example, instead of maximizing the high timing fidelity of UWB signals, many of the above discussed prior art tracking systems reduce fidelity by requiring synchronization between transmitter and receivers, which gives rise to synchronization errors. Those of skill in the art will appreciate the present invention that addresses the above and other problems.